"We found that patients who had vitamin D levels at the highest category had improved survival and improved progression-free survival, compared with patients in the lowest category," said lead author Dr. Kimmie Ng, an assistant professor of medicine at Harvard Medical School in Boston.

Those patients survived one-third longer than patients with low levels of vitamin D -- an average 32.6 months, compared with 24.5 months, the researchers found.

The report, scheduled for presentation this week at the Gastrointestinal Cancers Symposium in San Francisco, adds more weight to suspicions that vitamin D might be a valuable cancer-fighting supplement.

The study only found an association between vitamin D levels and colon cancer survival rates. It did not prove cause and effect.

Researchers for years have investigated vitamin D as a potential anti-cancer tool, but none of the findings have been strong enough to warrant a recommendation, said Dr. Len Lichtenfeld, deputy chief medical officer for the American Cancer Society.

"Everyone comes to the same conclusion -- yes, there may be some benefit, but we really need to study it carefully so we can be certain there aren't other factors that make vitamin D look better than it is," Lichtenfeld said. "These findings are interesting, and show that vitamin D may have a role in improving outcomes in cancer care."

Vitamin D is called the "sunshine vitamin" because human bodies produce it when the sun's ultraviolet rays strike the skin. It promotes the intestines' ability to absorb calcium and other important minerals, and is essential for maintaining strong, healthy bones, according to the U.S. National Institutes of Health.

But vitamin D also influences cellular function in ways that could be beneficial in treating cancer, Ng said. For example, she said it appears to reduce cell growth, promote the death of diseased cells, and inhibit the formation of new blood vessels to feed cancerous tumors.

The study authors found that certain types of cancer patients tended to have lower vitamin D levels. These included people whose blood specimens were drawn in the winter and spring months, people who live in the northern and northeastern states, older adults, blacks, overweight or obese people, and those who had lower physical activity and were in worse physical condition.

The patients were divided into five groups based on vitamin D levels, ranging from low to high. After adjusting for prognosis and healthy behaviors, the researchers found that patients in the group with the highest levels of vitamin D lived about eight months longer on average than those in the group with the lowest levels.

"We had a lot of information on their tumor, their treatment and their survival times, and their diet and lifestyle," Ng said. "That really allowed us to adjust for other potential factors that could influence what we're seeing."

It also took longer for cancer to progress in people with higher vitamin D levels -- an average 12.2 months compared with about 10 months in the group with the lowest. No significant differences were seen with regard to the type of therapy the patients received.

This increase in progression-free survival is the most compelling evidence indicating that vitamin D makes a difference in colon cancer, said Dr. Smitha Krishnamurthi, an associate professor of hematology and oncology at Case Western Reserve University School of Medicine in Cleveland.

"That is interesting because that's more of a cancer-specific endpoint as opposed to overall survival, which could be influenced by other factors like heart health," Krishnamurthi said.